A microphone array is a group of two or more microphones arranged in a specific geometric configuration and used to gather and process acoustic signals. One advantage of using a microphone array over a single microphone lies in the fact that the array adds dimensional information to the signal acquisition process. Accordingly, beam forming techniques may be used to provide a main lobe for receiving signals of interest that arrive from one or more desired directions. Beam forming increases the gain of the microphone array in one or more desired directions, while decreasing the gain in other directions to thereby improve the signal-to-noise ratio of a desired signal.
Adaptive microphone arrays may be configured to reduce side interference from sources of acoustic energy that are not situated in the main lobe of the array, as well as sources of acoustic energy that change position with reference to the array. Temporal and spatial information included in the signals collected by the microphone array are analyzed using array signal processing and adaptation procedures to formulate a filter transfer function for the array. The filter transfer function provides the microphone array with a fixed or dynamically changing directional pattern that reduces the response of the array to side interference. A null, or direction of minimum response, is provided in a particular direction along a specific fixed bearing, or along a set of specific bearings. The direction of minimum response may, but need not, change as a function of time. Due to the nature of commonly used adaptation algorithms that seek to minimize the error signal at an output of an adaptive filter, the adaptive microphone array tends to cancel a most significant source of interference that carries the most energy relative to other sources of interference. This cancellation may or may not be useful, depending on the geometry of the array, the locations of the sources of interference with respect to the array, and the number of sources of interference that exist. This approach may or may not provide sufficient cancellation of the sources of noise, especially in cases where enhancement for robust speech recognition is needed. In general, adaptive microphone arrays allow a coherent source of interference to be tracked, and if necessary, cancelled.